The present invention relates to oscillators used in electronic circuit, and more particularly to an oscillator for producing an oscillating signal of a desired frequency while reducing the magnitude and number of unwanted harmonics of the signal frequency.
Oscillators are conventional circuits used to generate an oscillating signal of a particular desired frequency. Oscillators are commonly used in electronic circuits for a multitude of purposes. For example, oscillators may be used in computer circuits to generate relatively high frequency, e.g., 16 MHz, clocking signals for computers. Along with the desired oscillating signal, an oscillator typically produces harmonics of the signal. These harmonics are produced by the nonlinear operation of transistors in the oscillator. Sometimes these harmonics interfere with the operation of other circuits in the vicinity of the oscillator. The harmonics are referred to as electromagnetic interference.
In conventional high-frequency oscillators, such as the aforementioned 16 MHz clocking oscillator for computers, short (fast) rise and fall times in the oscillator output signal are desired to obtain the relatively high output frequency. However, such short rise and fall times generate unwanted harmonics. Changing the oscillator circuit to lengthen, i.e., increase, the oscillator signal rise and fall times in an attempt to reduce the unwanted harmonics results in an undesired lowered oscillator frequency when the output signal waveform conforms to the input specification of logic circuits such as TTL or CMOS.
A number of other approaches have been used to reduce the amount of unwanted harmonics produced by oscillators while maintaining the desired output oscillator frequency. In some cases, filters have been added to conventional oscillators to filter out the harmonics. This approach has the disadvantage that various filters are necessary, depending upon the harmonic frequencies to be removed. In other cases, the circuitry of the oscillator itself has been changed in an attempt to reduce harmonics. This approach suffers the disadvantage that different types of oscillators need to be modified in different ways.
Thus, a simple, inexpensive, and relatively universal manner of reducing the harmonic output of various types of oscillators, particularly high-frequency oscillators, while maintaining the desired output oscillator frequency, has heretofore been lacking.